Flash is the one member where I think they're going to wait until they see how the TV series goes. Hell he might even be off the table completely once they decided to go ahead with the TV show. Same reason why Green Arrow is off the table.
If that's the case then you need a speedster. There are other options they can pick from. They might have to mine the Justice Society (Jesse Quick/Liberty Belle) or go for a lower tier Justice League member. I think Green Lantern might be off the table unless they're ready to relaunch the Green Lantern franchise.
Honestly if I had to pick I'd want the following line up.
Superman, Wonder Woman, Batman, Cyborg, Aquaman (I want Kaldur Aqualad to tag along), Blue Beetle (the Scarb would be interesting to mess around with. The Jamie version to have more diversity), and
Vixen (to have another woman on the team and to cover a wide varity of powers).